Patients with type 2 diabetes mellitus (T2DM) frequently develop cardiovascular complications which contribute significantly to morbidity and mortality. Insulin resistance and endothelial dysfunction are two cardinal features of T2DM. Whether microvascular insulin resistance contributes to the increased cardiovascular morbidity and mortality in patients with T2DM remains to be defined. T2DM is associated with humoral and inflammatory changes that can cause insulin resistance and endothelial dysfunction. Among them, elevation of plasma free fatty acids (FFAs) and over-activation of the renin-angiotensin system (RAS) are thought to play pivotal roles in vascular insulin resistance and endothelial dysfunction, and in the increased cardiovascular morbidity and mortality of diabetes. In the proposed studies, we will test the hypotheses that: 1) elevation of plasma FFAs blunts insulin-mediated cardiac and skeletal muscle microvascular perfusion and glucose uptake by activating inflammatory signaling and increasing endothelin 1 (ET- 1) secretion/action; 2) AT1R blockade increases cardiac and skeletal muscle microvascular perfusion, augments insulin-mediated glucose uptake, and attenuates FFA-induced dysfunction in the coronary and skeletal muscle microcirculation in healthy humans; and 3) patients with diabetes have decreased coronary microvascular flow reserve and blunted vasodilatory response upon insulin stimulation in the cardiac and skeletal muscle microvasculature, and that these abnormalities are corrected with anti-inflammatory therapy and/or AT1R blockade. We will quantify cardiac and skeletal muscle microvascular perfusion in healthy and diabetic humans and examine the potential mechanisms underlying microvascular insulin resistance. Results from the proposed studies should help to define the mechanisms underlying insulin resistance in cardiac and skeletal muscle microvasculature in diabetic humans and open a new avenue for future mechanistic, diagnostic and/or therapeutic studies. PUBLIC HEALTH RELEVANCE: Patients with type 2 diabetes are prone to suffer heart diseases such as heart attack and heart failure; both cause significant morbidity and mortality. The underlying mechanisms remain unclear. Patients with type 2 diabetes have decreased responses to insulin, a condition called insulin resistance. Insulin resistance has been implicated in the development of cardiovascular diseases in diabetic patients. Insulin increases blood flow, hence oxygen and nutrient delivery, to tissues, including heart and skeletal muscle. We and others have recently shown that insulin resistance is present in the small blood vessels in heart and skeletal muscle. Whether this contributes to the increased cardiovascular morbidity and mortality in persons with diabetes is not known. Diabetes is associated with many biochemical abnormalities which are capable of causing insulin resistance and abnormal vascular function in peripheral tissues. In this proposal, we plan to examine whether these abnormalities impair insulin action in the small vessels in human heart and skeletal muscle and whether patients with type 2 diabetes have an abnormal response to insulin in small blood vessels that nourish the heart and skeletal muscle. We will use state-of-the-art techniques to non- invasively measure small blood vessel perfusion in the human heart and skeletal muscle. Results from the proposed studies should shed light to our understanding of the relationship between type 2 diabetes and cardiovascular diseases in humans and open a new avenue for future mechanistic, diagnostic and/or therapeutic studies.